2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 11:05 AM


AMOROSO, Lee, Western Earth Surface Processes Team, U.S. Geological Survey, 2255 N. Gemini Drive, MS 7420, Flagstaff, AZ 86001 and MAHAN, Shannon, US Geol Survey, Box 25046 Federal Center, Denver, CO 80225, lamoroso@usgs.gov

Alluvial fans, derived from Paleozoic sedimentary rock units, are common along the Hurricane escarpment. Morphology, topographic position, and soil chronosequence were used to assign ages from middle-late Pleistocene to late Holocene. Most of the Holocene fluvial and associated minor debris flow deposits consist of unconsolidated gravel and boulders with little fine materials. In contrast, the Pleistocene alluvial fans consist of debris-flow deposits with minor slopewash and stream alluvium. Pedogenic carbonate accumulation is common; a calibrated carbonate rind chronosequence was used to estimate that many surficial fan deposits are > 45 ka. The initiation of the debris-flow dominated alluvial fan deposition is probably older than ~ 75 ± 40 ka, estimated using morphologic (diffusion) modeling of a faulted late Pleistocene fan. These fans, confined to locations near the escarpment base, are composed of partly to well-consolidated deposits of gravels, cobbles, and boulders; most deposits are matrix supported. The sand-size and finer matrix materials appear to be derived from higher on the hillslope rather than weathered in place or generated during downslope transport.

At approximately the Pleistocene-Holocene transition there may have been a change in transport process from dominantly debris-flow failure to fluvial. The matrix-rich debris flows may have been triggered by hillslope erosion during the wetter-to-drier climate transition. The source of this debris-flow material is weathered colluvium that likely mantled hillslopes during Pleistocene pluvial periods. The matrix-poor Holocene deposits suggest that much of this colluvial mantle was removed by the end of the Pleistocene. Buried soils (5 YR hue, silty and carbonate rich) were found overlying/within two of the five debris-flow deposits. The modern alluvium contains considerable silt and sand coming from the frequent dust-laden windstorms. Analogously, the buried soils have significant silt to very fine sand that may be of eolian origin. Others have suggested that eolian activity is related to changes in climate or sediment availability. Because the silts in the buried soil are probably eolian, Optically Stimulated Luminescence (OSL) dating is in progress to confirm the minimum age of the buried debris flow deposits.